Electromagnetic induction
| Electromagnetism |
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| Electricity · Magnetism · Magnetic permeability |
Electromagnetic induction is where a voltage or current is produced in a conductor by a changing magnetic flux. It may happen when a magnet is moved in a solenoid, or when a solenoid is constantly moved in a stationary magnetic field, thus changing the magnetic flux.
Magnetic flux
When a coiled wire is introduced near a magnet, the magnetic lines of force pass through the coil. This causes the magnetic flux to change. Magnetic flux is represented by the symbol [math]\displaystyle{ {\Phi} }[/math], therefore we can say that [math]\displaystyle{ {\Phi} }[/math] = BAcos(a) and the resulting unit will be [math]\displaystyle{ Tm^2 }[/math], where T is the unit for magnetic field and [math]\displaystyle{ m^2 }[/math] is the unit for area.
The changing magnetic flux generates an electromotive force (EMF). This force moves free electrons in a certain way, which constitute a current.
Faraday's law
Michael Faraday found that an electromotive force is generated when there is a change in magnetic flux in a conductor.
His laws state that:
[math]\displaystyle{ \mathcal{E} = {-{d\Phi} \over dt} }[/math]
where,
[math]\displaystyle{ \mathcal{E} }[/math] is the electromotive force, measured in volts;
[math]\displaystyle{ {d\Phi} }[/math] is the change in magnetic flux, measured in webers;
[math]\displaystyle{ dt }[/math] is the change in time, measured in seconds.
In the case of a solenoid:
[math]\displaystyle{ \mathcal{E} = {-N{d\Phi} \over dt} }[/math]
where,
N is the number of loops in the solenoid.
Lenz's law
The negative sign in both equation above is a result of Lenz's law, named after Heinrich Lenz. His law states that the electromotive force (EMF) produces a current that opposes the motion of the changing magnetic flux.
Electromagnetic Induction Media
Alternating electric current flows through the solenoid on the left, producing a changing magnetic field. This field causes, by electromagnetic induction, an electric current to flow in the wire loop on the right.
Faraday's experiment showing induction between coils of wire: The liquid battery (right) provides a current that flows through the small coil (A), creating a magnetic field. When the coils are stationary, no current is induced. But when the small coil is moved in or out of the large coil (B), the magnetic flux through the large coil changes, inducing a current which is detected by the galvanometer (G).
A diagram of Faraday's iron ring apparatus. Change in the magnetic flux of the left coil induces a current in the right coil.
A three-dimensional rendering of a solenoid.
The longitudinal cross section of a solenoid with a constant electrical current running through it. The magnetic field lines are indicated, with their direction shown by arrows. The magnetic flux corresponds to the 'density of field lines'. The magnetic flux is thus densest in the middle of the solenoid, and weakest outside of it.
Rectangular wire loop rotating at angular velocity ω in radially outward pointing magnetic field B of fixed magnitude. The circuit is completed by brushes making sliding contact with top and bottom discs, which have conducting rims. This is a simplified version of the drum generator.
Columbia iron vane type clamp-on ammeter
Hawkins Electrical Guide - Figure 292 - Eddy currents in a solid armature
Hawkins Electrical Guide - Figure 293 - Armature core with a few laminations showing effect on eddy currents
Small DC Motor pole laminations and overview
